1. Field of the Invention
The present invention relates to a rotary-percussion drill for forming holes in a stone material, preferably, having a diameter in a range from 3 to 45 mm, and which is used with a rotary-percussion hand-held power tool and has a four-start helical shaft.
2. Description of the Prior Art
Four-start helical shafts of rotary-percussion drills usually have a helix cross-section having a fourfold rotational and mirror symmetry, with radially projecting helical webs extending from concave groove bottoms, tapering radially outwardly, according to German Publication DE 4419641, or having the same width according to U.S. Pat. No. 2,673,716.
U.S. Pat. No. 4,749,051 discloses a four-start drill helical shaft with a helix cross-section having a fourfold rotational and mirror symmetry and in which radially projecting helical webs, which extend from concave groove bottoms, pass into envelope circle arcs of the helix envelope circle.
U.S. Pat. No. 2,895,355 discloses a four-staff helical shaft of a boring bar having a helix cross-section with a four-fold rotational and mirror symmetry with concave groove bottoms and convex arc-shaped helical webs which, according to German Patent No. 206,517 can be elliptically formed and have an undercut with respect to their web width.
The helix cross-section geometry of the drill helical shaft has a substantial influence on the spectrum of characteristics of a rotary percussion drill which, when used in an hand-held power tool, should meet numerous requirements. Without naming them all, at a predetermined boundary conditions (helix diameter, impact energy, torque, bending torque, drill material) of the used drill use, e.g., mass, volumetric displacement capacity, frictional coefficient, break resistance, acoustic impedance, torsional stiffness, and mean value, as well as variations of the bending stiffness, should all be matched with each other.
Accordingly, an object of the present invention is to provide a four-start helical shaft for a rotary-percussion drill which would have an optimal spectrum of characteristics for use with a hand-held power tool.